The present invention relates to vertical/short take-off and landing (V/STOL) aircraft and more particularly to the use of an articulated rotor propulsion system for providing VTOL and high-speed horizontal flight.
Aircraft having propellers or rotors located symmetrically on both sides of the aircraft and capable of being rotated 90 degrees upward from a horizontal axis position are known as tilt-rotor/tilt propeller aircraft. The rotors"" vertical axis positioning directs airflow downward and thrust upward and allows the aircraft to move in a vertical plane or to simply hover. Positioning the propellers"" or rotors"" axis horizontally directs the thrust forward and allows for conventional level flight. In this latter configuration, lift is provided by the flow of air around lifting surfaces such as wings or horizontal stabilizers. In between, the generally vertical and generally horizontal axis configuration, any tilt angle can be obtained, resulting in various corresponding oblique flight paths and acceleration-deceleration phases also known as forward and backwards transitions.
Such aircraft combine a helicopter""s slow flight capability with its ability to take off or land vertically and a conventional airplane""s capacity for high-speed level flight. An airplane can attain about twice the speed of a typical helicopter; over 300 miles per hour, opposed to less than 170 miles per hour for a helicopter. However, a typical prior art tilt-rotor aircraft""s inherent mechanical and structural complexity increases its empty weight over that of a similarly sized airplane while also increasing production cost. Furthermore, such an aircraft""s development in terms of aerodynamic qualities, stability, flight dynamics and control is made more difficult due to the aerodynamic effects at slow speeds and the mass of air displaced by the propeller around the aircraft""s various lifting surfaces. This mass of air is known as propeller wash or simply as prop wash.
Propellers or rotors are usually attached to engines mounted on either end of a wing which serves as a structural support. However, this setup causes a problem during the transition from vertical to horizontal flight. The changes in the angle of the direction of the prop wash do not vary directly with the changes in the angle at which the propellers are tilted. As long as the horizontal velocity and the resulting lift from the conventional lifting surfaces have relatively low values, most of the lift is provided by the propellers or rotors. Because of this phenomenon, the propellers will tilt forward very slowly as the aircraft begins a transition to level flight and then the propellers will tilt forward faster as the wings begin to provide more of the lift. On the contrary, the tilt angle of the prop wash changes rapidly at first but the change slows down as the aircraft completes transition from vertical to horizontal flight.
Up to this time, in the prior art, the following two configurations have been used to try to avoid the problems caused by the different rates at which the propellers and their resulting prop wash tilt with respect to the wing;
The horizontal wing is permanently fixed to the aircraft and the propellers rotate upwards. However, this configuration has a major disadvantage. When the propellers are in the vertical position, the prop wash acts directly downward onto the wing and creates a downward force which is analogous to negative lift. This loss in lift effectively reduces the aircraft""s useful load. In the other method, the horizontal wing is permanently fixed to the propellers and pivots with the propellers. This method eliminates the negative lift problem but the method causes another problem. When the wing is pivoted up and the aircraft still has horizontal velocity, the entire surface of the wing is placed directly in the path of the airflow. The mass of air suddenly hitting this huge flat surface causes stability and control problems and further causes the wing to stall, when the apparent angle of attack exceeds approximately 15 degrees.
Aircraft employing multiple rotors that are symmetric about the fuselage also suffer from asymmetric loading during vertical flight due to a differential in power between the rotors, engines or a transmission failure. Such asymmetric loading presents a dangerous condition and as a result, these aircraft have cross-coupling and redundant transmissions to prevent this condition. The aircraft, as a result, becomes complicated, heavier and expensive.
Helicopters are limited in forward speed by compressible effects on the advancing rotor and further by the drag on the hub and fuselage. Furthermore, helicopters suffer a power drain due to the tail rotor being used to counteract the torque about the fuselage that is caused by rotating the rotors. Additional losses occur due to the resultant circulation induced in the prop wash. In vertical flight, the prop wash impinges upon the fuselage and further reduces lift and thus useful load.
There are two solutions in the prior art which address the above problems. U.S. Pat. No. 3,049,320 to Fletcher and U.S. Pat. No. 5,758,844 to Cummings. Fletcher uses a ducted fan located at the center of gravity to avoid asymmetric propulsion loads. However, the use of a large shroud or duct creates drag, comparable to that of a tilt wing, during transition. When horizontal velocity is present, substantial drag is caused by the rotation of the duct from the horizontal axis. Fletcher, as a result of this drag, is limited to forward flight only when the ducted fan is aligned horizontally. Furthermore, the payload in Fletcher is located at least a distance of the rotor radius from the center of gravity, thus making vertical flight and hover control extremely sensitive to payload weight and the resulting aircraft is given a large gross weight to reduce this sensitivity.
Cummings also uses a ducted fan and thus suffers from the same disadvantages as Fletcher and in addition suffers the same disadvantages attributed to the tilt wing. In addition, Cummings has two disadvantages.
First, the ducted fan and motor of Cummings rotate forward from a vertical axis to a horizontal axis. As a result, the center of gravity changes substantially forward to a position necessary for horizontal flight. As a result, the aircraft would be substantially less stable in forward flight when the duct is aligned on a vertical axis. This is an important factor, because the diminished horizontal flight capabilities in this configuration creates a safety problem, and degrades short landing performance.
Second, Cummings suffers from additional drag in both horizontal and vertical flight because the prop wash impinges directly upon the fuselage. The configuration in Cummings substantially impairs forward visibility in horizontal flight if the aircraft was used as a piloted aircraft.
Fletcher, Cummings, and helicopters have the vertical center of gravity located beneath the rotor which makes the aircraft inherently unstable in vertical flight and hover.
Fletcher, Cummings and other ducted fans and jets have high exit velocities that dislodge and throw objects on the ground such as rocks and other debris. This high exit velocity is a result of the high disk loading. Although high disc loading is advantageous for high speed horizontal flight, it causes a variety of problems during vertical take-off and landing on unimproved landing areas including persons in the vicinity of such takeoffs and landings.
The present invention is a uniquely configured tilt rotor aircraft capable of vertical and short take-offs and landings (V/STOL) and has inherent features that increase the aircraft""s safety and performance. In horizontal flight, the aircraft uses a wing, as a lifting surface, coupled with either a horizontal tail or canard for longitudinal stability and control. A vertical tail is used for directional and roll stability with a conventional rudder. Furthermore, conventional control surfaces, such as ailerons, elevators and flaps, are used which are well known to those skilled in the art. The wing is a traditional form with a U shaped cut out in which a co-axial counter rotating rotor system is arranged.
In vertical flight or hover the aircraft is controlled by conventional cyclic pitch or by a plurality of deflectors located in the prop wash of the rotors. The location of the center of gravity above the rotor system coupled with the aerodynamic center of the vertical surface make the aircraft statically stable in the vertical flight and hover mode. The rotor system rotates from a vertical axis position through generally 90 degrees to a horizontal axis position. The rotor system provides vertical thrust in the vertical flight position and horizontal thrust in the horizontal position.
The fuselage is located generally directly above the forward half of the rotor system. The fuselage is supported from a forward connection to the leading edge of the wing. The location of the fuselage avoids the accelerated air flow of the prop wash. During vertical flight and hover, the fuselage is in the draw stream of the rotor system which has a higher velocity than the free stream, however, the velocity is substantially less than the velocity exiting the rotor system and therefore, the fuselage realizes less drag. In horizontal flight, the fuselage acts as an enlarged spinner directing the air stream to the outer regions of the rotor system where the majority of the thrust is produced. The cockpit is located in the fuselage. As a result of the fuselage""s location along the longitudinal axis, eccentric lateral loading is eliminated and further the effect on the center of gravity due to payload variations are minimized. The location of the cockpit also provides superior visibility for the pilot since the line of sight is unobstructed.
This minimization of the variation of the center of gravity is important during vertical flight because the movement of the center of gravity away from the center of lift of the rotor system requires a restoring moment either through cyclic pitch or deflecting vanes, each of which reduce the useable lifting thrust of the rotor system.
The aircraft uses conventional landing gear that allows the aircraft to take-off and land when the rotor system is aligned along the horizontal axis. The conventional landing gear capability improves the safety of the aircraft during emergencies and further allows the aircraft to reduce the fuel consumption experienced during vertical take-offs and landings.
The configuration of the aircraft allows for transfer between horizontal forward flight and vertical flight without moving the center of gravity and without creating substantial drag that reduces the lift/drag (L/D) ratio. Maintaining the L/D ratio provides favorable flight characteristic regardless of the orientation of the rotor system. This flight performance permits the aircraft to glide to an emergency landing without the risks associated with autogyration and stalling of the wings that is associated with tilt wings.
The location of the single rotor system avoids eccentric asymmetric thrust during power loss or engine failure, that are associated with tilt rotors that use symmetrically opposed rotor systems. The rotor system position eliminates the need for cross-coupling and additional redundant systems, thus keeping the aircraft simple, low weight and inexpensive as compared to other tilt rotor and tilt wing aircraft.
The co-axial counter rotating rotor system reduces the power requirement by eliminating the need for torque compensating means. The second rotor recaptures the circulation that is induced in the prop wash by the first rotor. The co-axial counter rotating rotors decrease the disc loading compared to a single rotor during vertical flight and hover. A brake system stops the second rotor during high-speed forward flight thereby increasing the effective disc loading.
A plurality of engines reduces the deleterious effects of an engine failure situation, thus further improving the safety of the aircraft.